SYNTHESIS OF PLA HOMO AND COPOLYMERS AND THEIR NANOCOMPOSITES FOR ADVANCED MATERIALS

During the past 20 years, a number of aliphatic polyesters have aroused considerable interest due to their biodegradability and biocompatibility, since their use would to reduce the quantity common non-biodegradable polymers (for example PET). In this scenario, studies (both academic and industrial) have been focused on one polymer in particular: polylactic acid (PLA). It is biodegradable, biocompatible and compostable and derives from renewable resources such as corn, potato, cane molasses and beet sugar; it belongs to the family of aliphatic polyesters commonly obtained from α-hydroxy acids which includes other kinds of polymers such as polyglicolic acid or polymandelic acid. PLA was synthesized for the first time by Carothers in 1932 and it was introduced in the market for medical applications at the end of 1960; in the last ten years its good mechanical properties, comparable to those of polystyrene, and an increasing interest for biodegradable polymers have made it interesting for others industrial applications especially in packaging applications as an environmental friendly substitute of traditional plastics. PLA is certainly the most promising and interesting biodegradable polymer for large scale applications for which could be assumed a gradual and progressive replacement of traditional materials coming from hydrocarbon, but there are many aspects such as thermal resistance, impact resistance, gas barrier properties that need to be improved because, at present, are lower than those of traditional polymers. The present PhD project aimed at the synthesis and the study of the properties of new biocompatible polymeric materials based on PLA, focusing the attention on to two main aspects that may have an effect on the final properties of the polymer: the control of the molecular architecture through the use of appropriate chain regulators able to modify the macromolecular structure and he use of nanoparticles, used as such or modified on the surface, added to the polymer matrix. Complex macromolecular architectures give to the materials special properties (for example low melt viscosity, shear sensitivity) that allow the increase of the fields of applications regarding to traditional polymers, while the possibility to use nanometric fillers could represent a new solution in composite materials field; in fact the high surface area of nanoparticle can reduce the amount of fillers added to the polymer improving the properties of the materials even with low percentage of mineral ( 5% w/w). In this work several PLAs with modified macromolecular architecture (star, tree and tree-star) have been synthesized to evaluate the effect of different structure on molecular and rheological properties. Different comonomers have been chosen to obtain complex structures. SEC analyses on samples synthesized with different comonomers show the effect of multifunctional comonomers on the molecular properties of the PLA, especially on the polydispersity index of the system. Star structures show a decrease of polydispersity index, while tree polymers have higher polydispersity than the one of linear PLA. Rheological analyses show the different viscosity of systems having complex macromolecular architectures in comparison with linear PLA. A more complex system is represented by tree-star polymer, obtained using a combination of two or more comonomers; these polymers have a very complex structure, which requires a careful control of feed, allowing to obtain a wide range of PLA with different rheological behavior. The molecular weights and the viscosity of the materials can be modulated changing the comonomers ratio obtaining a wide range of materials with different properties. Nanocomposites with percentages form 0.5% to 5% w/w of different fillers, montomorillonite and nanosilica, have been synthesized. The effects of nanoparticles on molecular properties of PLA are confirmed by the rheological analyses: increasing the amount of fillers complex viscosity decreases, but for low quantity (0.5% and 1% w/w) the viscosity of nanocoposites is higher than pure PLA even if the Mn values are lower. The use of nanoparticles can also modify thermal behavior of PLA; pure PLA has a very slow kinetic of crystallization that leads to a very low tendency of this polymer to form crystals during the cooling phase, therefore the material has an high amorphous phase with a clear and large glass transition. For this reason nanocomposites have been analyzed both with dynamic and in isothermal scanning to study the crystallization process, a very important aspect for the processing and the properties of the polymers, in order to evaluate difference from standard PLA. Nanosilica act as nucleating agent promoting the crystallization process of PLA, while materials containing Cloisite have a large amount of amorphous phase, probably because lamellas are not well separated, and only some samples crystallize during the cooling phase: but anyway the process has very low intensity and is very slow. The surface modification of nanoparticles can reduce the problem caused by the different surface energy between the organic phase (polymer) and the mineral phase (nanoparticle) but can also lead to a variation of material properties. Different organosilanes were used as coupling agents to modify the surface of nanoparticles. Modified nanoparticles were characterized with different techniques to determine both quantitatively and qualitatively the presence of silane on the particle. Titration was used to have quantitative results about the yield of surface modification reaction. Filler-polymer interaction and dispersion of the mineral is improved in presence of the coupling agents, as shown in TEM analysis. The modification of nanoparticles, in particular silica, promotes the crystallization process of the polymer; increasing the amount of silane higher crystallization temperature and crystallization heat are observed and also all crystallization processes are faster in presence of high quantity of silane due to a higher homogeneity of the system. TGA analyses show that all nanoparticles improve thermal stability of PLA. All samples have been synthesized without stabilizers to verify the action of fillers on thermal stability of PLA. The most interesting behavior has been observed in presence of pure silica with a very interesting increase of the degradation temperature. The use of modified silica also decreases the rate of the degradation. PLA nanocomposites have been used to prepare polymeric films by casting to evaluate the permeability of these materials towards different gases, like O2, CO2 and water vapor, in order to evaluate the effect of crystallynity and nanoparticles on this property.